Research Communications Enantiomer Fractions Are Preferred to Enantiomer Ratios for Describing Chiral Signatures in Environmental Analysis T O M H A R N E R , * ,† K A R I N W I B E R G , ‡ A N D ROSS NORSTROM§ Atmospheric Environmental Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada, Department of Chemistry, Environmental Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden, and Environment Canada, Hull, Quebec K1A 0H3, Canada
The enantiomer ratio (ER) is currently the standard descriptor of enantiomeric (chiral) signatures for environmental samples. In this paper, we argue for the adoption of the enantiomer fraction (EF) as the standard descriptor by showing drawbacks to the use of ER. The enantiomer fraction is superior because it provides a more meaningful representation of graphical data and is more easily employed in mathematical fate expressions. Several useful expressions are presented that allow EF to be used for tracking and apportioning chemical movement between environmental compartments and for investigating microbial degradation processes.
Introduction Chiral analysis is becoming increasingly popular in the field of environmental science for investigating the transport and fate of chemicals in various media (1). This technique has recently been applied to environmental samples to yield information on air-water exchange of R-HCHs in oceans (2) and lakes (3), the revolatilization of pesticides from soils (4), and the importance of microbial degradation in controlling environmental lifetimes of persistent chemicals (5). Analysis of chiral compounds in various biological compartments may provide valuable insight to how chemicals are accumulated, degraded, and translocated within food chains (6). The property of chirality is attributed to a compound if it can exist as two non-superimposable mirror image formss similar to our left and right hands. These two forms are designated as (+) and (-) enantiomers based on their interaction with plane-polarized light. Chiral compounds of environmental significance include R-hexachlorocyclohexane (R-HCH), cis- and trans-chlordane, o,p′-DDT, heptachlor, and heptachlor-exo-epoxide (HEPX). Some PCBs and their metabolites (e.g., methylsulfone PCBs) exhibit axial chirality (atropisomerism) due to hindered rotation about the biphenyl σ-bond (6). Physical processes are not able to distinguish between the two enantiomeric forms of a compound. Consequently, chiral chemicals are almost always produced as a racemate in which 50% of the compound is * Corresponding author e-mail:
[email protected]. † Atmospheric Environmental Service. ‡ Umea ˚ University. § Environment Canada. 218
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the (+) form and 50% is the (-) form. In the environment, the racemic signature remains unchanged by physical removal mechanisms such as hydrolysis and photolysis reactions. However, the mechanisms of microbial degradation and biological metabolism may be enantioselective and thus alter the enantiomer signature. Enantioselective permeability through biological membranes has also been indicated. Totally selective transfer of (+)-R-HCH across the blood-brain barrier occurs in seals and rats, whereas the ER in blubber is between 1 and 2 (7, 8). This altered signature or “fingerprint” can be exploited to track a compound’s movement and transformation. Chromatography, using a chiral stationary phase, is able to separate the (+) and (-) enantiomers in environmental samples. Until now, the most popular way for describing this altered signature was to use the concept of enantiomer ratio (ER) where
ER ) A+/A[A+ and A- correspond the peak areas of the (+) and (-) enantiomers; equal molar response factors are assumed]. The ER in the sample is often compared to the value in a standard that is typically racemic, i.e., ER ) 1.0. However, there are several limitations to using ER. When used graphically, the ER results in misleading representation of data. Because of the way it is defined, the ER can range from 0 to infinity. The ER of R-HCH in seal brain is approximately infinity, and it is therefore not possible to represent it graphically (7). Therefore, a unit change in ER away from unity in the downward direction (i.e.,
